1. Technical Field
The invention relates to a motor vehicle having a deceleration warning device for high negative accelerations. The invention also relates to a method for activating a warning signal on a motor vehicle.
2. Background Art
A considerable number of road traffic accidents fall into the category of rear-end collisions. In the context of the worldwide initiative for accident reduction such driving situations, or at least their results, could be significantly reduced if the occurrence of a deceleration or a braking process were displayed in an appropriate manner to the following vehicle.
Although it is usual to utilize brake lights, which indicate actuation of the vehicle brakes to following traffic, in braking systems of motor vehicles, these brake lights give only binary information on a braking process and do not indicate its intensity. This means that while the driver of a following vehicle sees that a braking process is being initiated in the vehicle ahead, he does not know how heavy the braking is.
A braking system which could indicate abrupt braking as supplementary information to the following traffic, and could therefore indicate a safety-critical situation at an early stage, would enable the driver of the following vehicle to react adequately to the danger situation more quickly.
Such warning systems have by now been implemented by various manufacturers in their vehicles, but do not yet offer an optimum compromise between system availability and potentially incorrect warnings. In some cases these systems not only come into operation in emergency braking actions, but signal that a vehicle is approaching the limit region. This can result in corresponding dissatisfaction among vehicle drivers and customers if assistance systems intervene too often or too early. In addition, there is a risk of incorrect reactions by other road users as a result of premature warning of deceleration.
The document DE 297 09 110 U1 relates to a warning device for motor vehicles having ABS braking systems which includes a warning transmitter which is activated by intervention of the ABS system in the braking process. Thus, a deceleration warning is known from DE 297 09 110 U1 which has detection of an ABS control action as a necessary precondition. This has significant disadvantages for its operation. For example, in the event of low friction values (e.g. on snow or ice), among other situations, incorrect warnings may occur because an ABS system is activated by braking actions which, in terms of deceleration, in no way represent emergency braking. Furthermore, in traffic conditions with high friction values of the underlying surface (e.g. dry road) there is no possibility of issuing a deceleration warning even for critical vehicle decelerations unless the ABS is activated. Thus, entirely normal ABS settings permit significant deceleration values without intervention by the ABS.
A further deceleration device is disclosed by DE 100 05 867 A1. The warning system known from this document emits warning signals when hazards are recognized. The warning system is equipped with electronic circuits and sensors which detect the vehicle's movement and the traffic situation to the rear, permitting conclusions to be drawn regarding the level of potential risk. The device described in this document presupposes simultaneous monitoring of the movement of the vehicle itself and of traffic to the rear, which monitoring must be effected by an appropriate sensor system. A hazardous situation is deduced from the difference between the vehicle's own movement and that of traffic to the rear, and corresponding warning signals are emitted. Any brake control system (ABS, ESP) present in the vehicle is taken into account only to the extent that activation thereof is included in the assessment of the risk potential. Deceleration warnings in the event of high negative accelerations without the intervention of ABS/ESP are not made possible, so that incorrect warnings in the event of low friction values are possible.
Finally, EP 0 611 679 B1 discloses a deceleration warning device in which a “total risk value” formed by means of time and/or distance integrals is generated for the warning. The intensity of the deceleration warning is then scaled with reference to the total risk value. Braking processes as a result of engine braking (closing the throttle) without actuation of the brake pedal, are also utilized in activating the deceleration warning. However, the high demands of the implementation proposed in that document, which are reflected in corresponding demands on the memory and computation capacity of a control unit, are to be regarded as disadvantageous.